.\"	$OpenBSD: BIO_set_callback.3,v 1.4 2016/11/20 01:22:28 schwarze Exp $
.\"	OpenSSL 99d63d46 Oct 26 13:56:48 2016 -0400
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.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
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.Dd $Mdocdate: November 20 2016 $
.Dt BIO_SET_CALLBACK 3
.Os
.Sh NAME
.Nm BIO_set_callback ,
.Nm BIO_get_callback ,
.Nm BIO_set_callback_arg ,
.Nm BIO_get_callback_arg ,
.Nm BIO_debug_callback
.Nd BIO callback functions
.Sh SYNOPSIS
.In openssl/bio.h
.Ft void
.Fo BIO_set_callback
.Fa "BIO *b"
.Fa "BIO_callback_fn cb"
.Fc
.Ft BIO_callback_fn
.Fo BIO_get_callback
.Fa "BIO *b"
.Fc
.Ft void
.Fo BIO_set_callback_arg
.Fa "BIO *b"
.Fa "char *arg"
.Fc
.Ft char *
.Fo BIO_get_callback_arg
.Fa "const BIO *b"
.Fc
.Ft long
.Fo BIO_debug_callback
.Fa "BIO *bio"
.Fa "int oper"
.Fa "const char *argp"
.Fa "int argi"
.Fa "long argl"
.Fa "long ret"
.Fc
.Ft typedef long
.Fo "(*BIO_callback_fn)"
.Fa "BIO *b"
.Fa "int oper"
.Fa "const char *argp"
.Fa "int argi"
.Fa "long argl"
.Fa "long ret"
.Fc
.Sh DESCRIPTION
.Fn BIO_set_callback
and
.Fn BIO_get_callback
set and retrieve the BIO callback.
The callback is called during most high level BIO operations.
It can be used for debugging purposes to trace operations on a BIO
or to modify its operation.
.Pp
.Fn BIO_set_callback_arg
and
.Fn BIO_get_callback_arg
set and retrieve an argument for use in the callback.
.Pp
.Fn BIO_debug_callback
is a standard debugging callback which prints
out information relating to each BIO operation.
If the callback argument is set, it is interpreted as a BIO
to send the information to, otherwise stderr is used.
.Pp
.Fn BIO_callback_fn
is the type of the callback function.
The meaning of each argument is described below.
.Pp
The BIO the callback is attached to is passed in
.Fa b .
.Pp
.Fa oper
is set to the operation being performed.
For some operations the callback is called twice,
once before and once after the actual operation.
The latter case has
.Fa oper
or'ed with
.Dv BIO_CB_RETURN .
.Pp
The meaning of the arguments
.Fa argp ,
.Fa argi
and
.Fa argl
depends on the value of
.Fa oper
(i.e. the operation being performed).
.Pp
.Fa ret
is the return value that would be returned to the application
if no callback were present.
The actual value returned is the return value of the callback itself.
In the case of callbacks called before the actual BIO operation,
1 is placed in
.Fa ret .
If the return value is not positive, it will be immediately returned to
the application and the BIO operation will not be performed.
.Pp
The callback should normally simply return
.Fa ret
when it has finished processing, unless it specifically wishes
to modify the value returned to the application.
.Ss Callback operations
.Bl -tag -width Ds
.It Fn BIO_free b
.Fn callback b BIO_CB_FREE NULL 0L 0L 1L
is called before the free operation.
.It Fn BIO_read b out outl
.Fn callback b BIO_CB_READ out outl 0L 1L
is called before the read and
.Fn callback b BIO_CB_READ|BIO_CB_RETURN out outl 0L ret
after.
.It Fn BIO_write b in inl
.Fn callback b BIO_CB_WRITE in inl 0L 1L
is called before the write and
.Fn callback b BIO_CB_WRITE|BIO_CB_RETURN in inl 0L ret
after.
.It Fn BIO_gets b out outl
.Fn callback b BIO_CB_GETS out outl 0L 1L
is called before the operation and
.Fn callback b BIO_CB_GETS|BIO_CB_RETURN out outl 0L ret
after.
.It Fn BIO_puts b in
.Fn callback b BIO_CB_WRITE in 0 0L 1L
is called before the operation and
.Fn callback b BIO_CB_WRITE|BIO_CB_RETURN in 0 0L ret
after.
.It Fn BIO_ctrl b oper larg parg
.Fn callback b BIO_CB_CTRL parg oper larg 1L
is called before the call and
.Fn callback b BIO_CB_CTRL|BIO_CB_RETURN parg oper larg ret
after.
.El
.Sh EXAMPLES
The
.Fn BIO_debug_callback
function is a good example.
Its source is in the file
.Pa crypto/bio/bio_cb.c .
.Sh SEE ALSO
.Xr BIO_new 3
